How popular is Syntha?

Syntha is a rare first name for women. Syntha is also a unique last name for both adults and children. (2000 U.S. Census)

Which version is better?

with its source form and related girl names. Cinthia, Cyntha, Cynthia(#538 from current records) and Synthia are the popular related forms of Syntha (outside Top 2000). Usage of these forms of Syntha was at its peak 7 decades ago (average #1298) and is now significantly lower, with the version Cynthia becoming less trendy. (Top 2000 Names, 2015)